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Locard Exchange Principle Visualizer

Run the Locard Visualizer Fullscreen

About This MicroSim

Edmond Locard's Exchange Principle states that "every contact leaves a trace." When two objects touch, material moves in both directions — and that two-way transfer is what crime-scene investigators collect and analyze.

This MicroSim turns the principle into a predict-then-check experiment. You pick a contact scenario, predict which traces will move from each object to the other, press Make Contact, and then verify your prediction against the evidence inventory that builds up on each object.

How to Use It

  1. Choose a scenario from the dropdown (for example, Suspect / Carpet).
  2. Read the prediction prompt and decide what will transfer each way.
  3. Press Make Contact and watch the colored traces stream in both directions.
  4. Read the Evidence Inventory to see exactly what landed on each object and roughly how much.
  5. Check the "What the lab would look for" panel to learn which analytical technique detects each trace type.
  6. Toggle Show trace labels on or off, and press Reset Scene to try again.

What You Can Learn

  • Apply Locard's principle to predict two-way trace transfer in new situations.
  • Connect specific trace types (fibers, skin cells, soil, residues) to the lab methods that identify them.
  • Explain why securing a scene quickly preserves fragile transferred evidence.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/locard-exchange-visualizer/main.html"
        width="100%" height="552" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Apply (L3) — predict and verify trace transfer.

Predict-Test-Observe (the core routine). For each scenario, students first write down their prediction for both directions of transfer, then press Make Contact to test it. The prediction step is what pushes this activity from "watch an animation" to "apply a principle."

Guided questions:

  • In the Shoe sole / Muddy soil scenario, name one trace that moves each way.
  • Which scenario produces evidence requiring DNA analysis? Which requires pollen analysis (palynology)?
  • A suspect claims they were never in the carpeted room. What two-way evidence would contradict that claim?

Extension. Ask students to invent a fifth scenario (e.g., bicycle tire / gravel) and predict the bidirectional transfer and the lab methods that would detect it.

References

Specification

This MicroSim was generated from a specification in Chapter 1: Foundations of Forensic Science and Legal Principles.

Design note: the specification mentions draggable objects. To keep the focus on the learning objective — predicting the two-way transfer — the objects are fixed and the Make Contact button animates the contact event itself. Dragging was omitted because repositioning the objects does not change what transfers between them.